1. Field of the Invention
The present invention relates to optical systems, and, more particularly, to optical wavelength division multiplexed systems in which channels are multiplexed and demultiplexed.
2. Description of the Related Art
An optical wavelength division multiplexed (WDM) system is an optical system carrying many different wavelengths, or frequencies, of light. The frequencies are closely spaced, and, from an information systems perspective, are also referred to as channels, which channels carry information.
In WDM systems as described above, optical filters, such as optical wavelength demultiplexers, have a very important role. One configuration of the channel dropping systems is shown in FIG. 1 where channels having predetermined frequencies are dropped by a filter 10 from the main signal stream of light A into another light path C (referred to as the dropped channel(s)), and all other channels are transmitted along path B (referred to as the transmitted channels). In this system, the frequencies of the dropped channels are predetermined. If channels at arbitrary frequencies can be dropped, more flexible systems will be built. One of the interesting filters is a wavelength tunable filter, an example of which is shown in FIG. 2. As shown in FIG. 2, wavelength tunable filter 12 drops (or separates out) arbitrary frequencies from the main signal stream of light A to another light path C (referred to as the dropped channel), and transmits the remaining frequencies along path B (referred to as the transmitted channels). An interesting tunable filter is an acousto-optic waveguide filter, which filters frequencies of light from the main stream in response to electric power provided to the acousto-optical waveguide filter. The frequencies of light which are dropped from the main stream are determined by acoustic frequencies which are applied to the device, and, accordingly, may be dynamically altered. If more than one acoustic frequency is applied simultaneously to the wavelength tunable filter, all of the corresponding channels are dropped.
With an acousto-optic waveguide filter, shown in FIG. 2, light wave A, which includes 4 frequencies (or channels) 1, 2, 3, and 4, is input to wavelength tunable filter 12. Wavelength tunable filter 12 drops channel number 3 into path C, while transmitting remaining channels 1, 2, and 4 along output path B. If the wavelength tunable filter 12 of FIG. 2 were, for example, an acousto-optic tunable filter, then the channel dropped along path C would be in response to an output of an acoustic wave generator 14 provided in the filter.
As channels in WDM systems become more closely spaced, demultiplexers have increasing difficultly isolating one or more channels from the other input channels and extracting the selected channel(s). Because the channels are so closely spaced in prior art WDM systems, problems with filtering shape and crosstalk (or leaked light) affect the WDM systems, as do problems with instability of optical power in adjacent channels. Most troublesome in WDM systems of the prior art are problems of crosstalk and instability of optical power in adjacent channels. Problems of crosstalk and instability of optical power most seriously affect the transmitted light, and problems of crosstalk most seriously affect the dropped light, as explained with reference to FIGS. 3A, 3B, 3C, 3D, 3E, 4, and 5.
Problems of filtering shape and of crosstalk in the context of filtering and dropping channels are explained with reference to FIGS. 3A-3E, which respectively show spectrums of light for input channels from which channel 3 is dropped.
FIG. 3A shows an ideal filtering shape surrounding channel 3. An ideal filtering shape F is rectangular, having a flattened top and a base of equal width W with the top of the filtering shape (as shown in FIG. 3B).
However, because of imperfections in conventional filters, an ideal filtering shape is difficult to achieve. Three common problems which occur with conventional filters include providing a filtering shape F with a top that is not flat (FIG. 3C), a filtering shape F with a base (W) considerably wider than the top of the filtering shape F (FIG. 3D), and a filtering shape F filtering an input light wave shifted by a distance S from the center of the spectrum (FIG. 3E).
The example of the filtering shape shown in FIGS. 3C results in a reduced amount of power present in channel 3, and, further, an altered shape of the optical spectrum in channel 3, after filtering. As shown in FIG. 3C, area C has been cut off of channel 3 by the filter F. To compensate for this loss of power, the width of the filter F may be expanded, resulting in crosstalk from adjacent channels 2 and 4, as shown in FIG. 3D. Of course, the shape of the filter F shown in FIG. 3D could have also resulted simply from imperfections in filter F which has a wider base than the top width. Crosstalk C in channel 3 resulting from either of adjacent channels 2 or 4 (crosstalk C is shown in FIG. 3E resulting from adjacent channel 2) also occurs when the center of the spectrum of the input light for channel 3 is shifted a distance S from the center of the spectrum of the filter F. Because of the close proximity of adjacent channels 2 and 4 to channel 3, channel 3 receives crosstalk C upon being filtered.
FIG. 4 shows a spectrum of light transmitted along path B of FIG. 2. If, in the example shown in FIG. 2, channel number 3 is dropped to path C, the output stream B should include only remaining channel numbers 1, 2, and 4. However, as shown in the spectrum of transmitted light of FIG. 4, a small portion of the light in channel number 3 remains and is transmitted along path B. This transmission of a small portion of the light in channel number 3 along path B, even though channel number 3 was dropped into path C, is due to insufficient isolation of channel 3 from channels 1, 2, and 4.
Also as shown in FIG. 4, the optical powers in channels 2 and 4, which are adjacent to channel 3, fluctuate in time and, therefore, provide channels 2 and 4 with instability in their signals. The problem of instability of the optical power due to the dropped channel (such as channel 3) is most acute in the channels adjacent (such as channels 2 and 4) to the dropped channel, but the instability also affects non-adjacent channels (such as channel 1), with decreasing intensity as the distance (in terms of frequency) from the dropped channel increases.
FIG. 5 shows a spectrum of the dropped light transmitted along path C of FIG. 2. As shown in FIG. 5, a problem arises with the dropped channel 3 transmitted along path C because the light dropped into path C includes light power contributions not only from channel 3 (which are desired) but, also, light power contributions from channels other than channel 3 (such as channels 1, 2, and 4). Light power from channels 1, 2, and 4 is undesired and is leaked along path C. The problem of the leaked optical power into the dropped channel (such as channel 3) is most acute in the channels adjacent (such as channels 2 and 4) to the dropped channel, but the leak also occurs in non-adjacent channels (such as channel 1), with decreasing intensity as the distance (in terms of frequency) from the dropped channel increases.
All of the above-mentioned problems are caused by interference between the channels. As shown in FIGS. 4 and 5, and as explained above, the effects on and of channel number 1 are weaker, because the channel position of channel 1 is further from the position of channel 3. A typical isolation of an acousto-optic waveguide filter is ˜20 dB for the adjacent channels when the channel spacing is 0.8 nm.
Therefore, filters providing increased isolation between the channels of the input light are desired. Also desired is a WDM system having increased isolation between the channels of the input light, but being of low cost and providing low optical loss.